The present invention is directed to control devices for bicycle transmissions and, more particularly, to an automatic shift control device that shifts the bicycle transmission in accordance with the speed of the bicycle.
Known bicycles are equipped with shifting mechanisms (transmissions) that can be shifted into a plurality of speed steps. The shifting mechanisms may be external shifting mechanisms or internal shifting mechanisms. An external shifting mechanism comprises, for example, a hub cog having a plurality of sprockets mounted on the rear wheel and a derailleur for moving a chain between the sprockets. An internal shifting mechanism has the shifting mechanism mounted inside the rear wheel hub. Both types of shifting mechanisms are usually connected with the aid of shifting cables to shift levers attached to handlebars or the like, and the optimum speed step suited to the riding conditions can be selected by manually operating the shift levers.
Shift levers are often placed close to the brake levers on the handlebar. During deceleration, the brake levers and the shift levers must be operated simultaneously. Because of the close proximity of the brake lever and the shifting lever, operation of the shift lever at such times is often impaired. In view of this, a shift control device for automatically switching speed steps in accordance with the speed of a bicycle has been proposed in Japanese Laid-Open Patent Application 8-113131. In this shift control device, the shift timing is varied depending on the magnitude of acceleration or deceleration. The goal is to speed up the shift timing during fast acceleration or deceleration, thus reducing rider discomfort. To construct such a shift control device, acceleration sensors and routines for calculating acceleration on the basis of speed must be provided. Consequently, there is a possibility that complicated automatic shift control will be involved and that the control response will be slower. Control procedures are further complicated because the speed steps must be determined on the basis of complex two-dimensional maps when both speed and acceleration are used. In addition, chattering may occur when rapid acceleration or deceleration occurs near a shift point as a result of the shifting device rapidly shifting back and forth between two adjacent gears.